The uptake of liposomes of defined composition by cells in vivo and in vitro provides a useful experimental method for modifying cellular composition and for introducing biologically active materials into different compartments of the cell. Fusion of liposomes with the plasma membrane and/or exchange of components between the plasma membrane and liposomes adsorbed to the cell surface will be used to introduce specific phospholipids, glycolipids and cholesterol into the plasma membrane of normal and neoplastic cells in vitro to investigate the role of these components in the expression of specific cell surface properties and their effect on cell growth behavior in vitro and in vivo. In other, similar experiments "membrane-vesicles" isolated from the plasma membrane of tumor cells will be fused with various acceptor cells. This fusion will permit transfer of plasma membrane proteins and glycoproteins between neoplastic cells exhibiting differing invasive and metastatic behavior so that the contribution of particular membrane proteins and glycoproteins in determining specific behavioral characteristics may be identified. These experiments will provide information on how changes in plasma membrane composition and cell surface organization in neoplastic cells are related to the altered behavioral properties displayed by these cells. Experiments are also proposed to evaluate the feasibility of using liposomes containing encapsulated lymphokines as a method for activating macrophage populations in tumor-bearing animals to augment host defense against tumor cells.